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1# Cumulative Kconfig recursive issue2# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~3#4# Test with:5#6# make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-02 allnoconfig7#8# The recursive limitations with Kconfig has some non intuitive implications on9# kconfig semantics which are documented here. One known practical implication10# of the recursive limitation is that drivers cannot negate features from other11# drivers if they share a common core requirement and use disjoint semantics to12# annotate those requirements, ie, some drivers use "depends on" while others13# use "select". For instance it means if a driver A and driver B share the same14# core requirement, and one uses "select" while the other uses "depends on" to15# annotate this, all features that driver A selects cannot now be negated by16# driver B.17#18# A perhaps not so obvious implication of this is that, if semantics on these19# core requirements are not carefully synced, as drivers evolve features20# they select or depend on end up becoming shared requirements which cannot be21# negated by other drivers.22#23# The example provided in Documentation/kbuild/Kconfig.recursion-issue-0224# describes a simple driver core layout of example features a kernel might25# have. Let's assume we have some CORE functionality, then the kernel has a26# series of bells and whistles it desires to implement, its not so advanced so27# it only supports bells at this time: CORE_BELL_A and CORE_BELL_B. If28# CORE_BELL_A has some advanced feature CORE_BELL_A_ADVANCED which selects29# CORE_BELL_A then CORE_BELL_A ends up becoming a common BELL feature which30# other bells in the system cannot negate. The reason for this issue is31# due to the disjoint use of semantics on expressing each bell's relationship32# with CORE, one uses "depends on" while the other uses "select". Another33# more important reason is that kconfig does not check for dependencies listed34# under 'select' for a symbol, when such symbols are selected kconfig them35# as mandatory required symbols. For more details on the heavy handed nature36# of select refer to Documentation/kbuild/Kconfig.select-break37#38# To fix this the "depends on CORE" must be changed to "select CORE", or the39# "select CORE" must be changed to "depends on CORE".40#41# For an example real world scenario issue refer to the attempt to remove42# "select FW_LOADER" [0], in the end the simple alternative solution to this43# problem consisted on matching semantics with newly introduced features.44#45# [0] https://lore.kernel.org/r/1432241149-8762-1-git-send-email-mcgrof@do-not-panic.com46 47mainmenu "Simple example to demo cumulative kconfig recursive dependency implication"48 49config CORE50 tristate51 52config CORE_BELL_A53 tristate54 depends on CORE55 56config CORE_BELL_A_ADVANCED57 tristate58 select CORE_BELL_A59 60config CORE_BELL_B61 tristate62 depends on !CORE_BELL_A63 select CORE64